Austrian Pat. PS No. 181590 discloses a zigzag folding device for folding textile webs, that is, heavy and elastic material. In that device the web runs over drawing rolls, operating in opposite directions, which are provided in a carriage located below the stack. The driven drawing rolls pull the web and move it against the bottom surface of the stack. The rollers thus serve as feed rolls and hold the web under tension. Due to the fact that they rotate in opposite directions the drawing rolls must be mounted in the carriage in such a manner that they can be pivoted up and down to ensure that only one drawing roll is in contact with the bottom surface of the stack at any time. As thus, at all times one of the two drawing rolls is not in contact with the stack, the stack is not adequately supported in the area of the drawing rolls. At the reversing points of the carrage trave; the web detaches itself from one drawing roll and its other side is brought into contact with the other drawing roll. As the latter is driven such that it rotates continually, the web is accelerated in the instant in which it makes contact with the rotating drawing roll: this causes a shock-type tensile stress, which does not damage elastic webs but may cause damage to inelastic webs, particularly thin ones. During this change from one drawing roll to the other the web can also move backward to some extent, as it is not supported by either drawing roll during this changeover. The reverse motion of the web causes imprecise determination of the point where the fold is made. For the intended purpose of this known device, i.e., the folding of textile webs, this is not required. However, this known device is not suitable for processing without damaging them either thin webs or paper webs which have to be folded at a specific weakened line.
Furthermore, German Pat. OS No. 24 02 027 discloses a device for folding endless webs. In this device, two rotationally driven conveyor belts are located in a stationary position below an oscillating stack. An opening gap for the web is provided between these conveyor belts. Two feed rolls, serving as a feeding device, are located below this opening. The feed rolls push the web up through the opening towards a board or the bottom of the stack. The web is grasped by the conveyor belts and is then clamped between these conveyor belts and the board, or the bottom of the stack. In the extreme positions of the stack, the conveyor belts are reversed. The reversal causes the formation of undesirable wrinkles, especially in thin material. In addition, this device does not allow the formation of a fold at precisely predetermined locations, e.g., at the location of a weakened line.